2. Field of the Invention
This invention relates in general to a pulse Doppler radar system with a range gate circuit and with a fixed target echo rejection circuit consisting of recursion filters with at least one delay element wherein a return loop is connected across the delay element and weighting changes of the recursion signal occur upon a change of the pulse repetition frequency (PRF) by switching weighting factors to a weighting multiplier.
2. Description of the Prior Art
When the changeover of the pulse repetition frequency occurs in radar devices, significant effects on signal processing occurs, as for example, in the suppression of fixed target echoes in the fixed target echo rejection circuit (MTI filter). The scanning filters of a fixed target echo rejection circuit form a transmission quadripole. If despite variation of scanning rates in the time domain specific transmission properties are to remain constant for example, the unstandardized lower cutoff frequency, the transmission behaviour corresponding to the scanning rate must be changed. When recursive scanning filters are used, this means that the weighting factors in the individual recursions must be varied according to the respective scanning rate. This causes a change of the standardized transmission characteristics. The transmission properties of the quadripole causes a specific transient response to such changes of the pulse repetition frequency.
FIG. 1 illustrates a known doubly recursive MTI filter with switchable weighting factors. The input signals of the recursive filter are supplied to a sample and hold circuit SH and are then supplied through a summing circuit SU to two series connected delay elements, VZG and VZG each of which are provided with a return loop. A first weighting amplifier WM is inserted in the return for the first delay element and is labelled channel A and weighting signals a1, a2 . . . are supplied to the weighting amplifier in channel A and the output is supplied to the summing circuit SU. A second channel B receives the output of the second delay element VZG and supplies its input to a weighting multiplier WM which also receives weighting factors b1, b2 . . . and channel B supplies its ouput to the summing circuit SU. The changeover of the weighting factors a and b occur as a function of the frequency selection of the radar pulses and thus these changeovers are made by the frequency selector circuit which also changes the pulse repetition frequency to the sample and hold circuit. The effect of the changeover of the weighting factors can be mathematically reproduced by means of an equivalent step function wherein the input signal function is additively changed. The amplitude of the step is calculated at EQU step=(a.sub.84 -a.sub..mu.).multidot.A+(b.sub.84 -b.sub..mu.).multidot.B+(c.sub..nu. -c.sub..mu.).multidot.C+ . . .
Upon occurrence of such a step at the input of the recursive filter, transient processes occur of the output magnitude, size, duration and spectral components of the transient depending on the amplitude of the step and on the transfer characteristics of the recursive filter. Spectral components which were not present in the input signal are generated for a specific time and fall with high probability into frequency ranges in radar devices which could be moving targets. Such spurious signals result in false alarms in radar devices for the duration of their occurrence.
Such disturbant influences can be avoided by interrupting the signal flow at the output of the recursion filter for the time duration of the transient process. A solution in this manner, however, has the disadvantage that useful signals resulting from the moving target echoes occurring during this time would be lost.